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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 7, Misalignment

Section 22, Determining the True Vibration Mode of Overhung Rotors

Determining how a rotor is shaking (mode) by phase analysis is relatively straightforward on rotors that have bearings at each end of the rotor. But for overhung (cantilevered) rotors, such as overhung fans and pumps, an extra precaution must be taken. By using this precaution, overhung rotor phase analysis becomes the same as for non-overhung rotors. The situation described is much more common on pumps; however, the same applies for any overhung rotor.

Probably the best way to explain this is to compare the phase reading on two similar overhung pumps. (For simplicity, look at the shaking mode of the pump only, although in actual practice the drive unit's shaking mode is also involved.) Notice that in the above diagram, both pumps show in-phase vibration modes in both their horizontal and vertical directions. Most often this leads the analyst to assume that in both situations the 1 x rpm vibratory sources are impeller unbalances. But now notice what possible change can occur if additional phase readings are taken on the suction pipe flange (even though there is no bearing at that location).

By comparing the vibration phase readings at Bearing A with the phase readings at Point C, Pump #1 continues to show in-phase vertical and in-phase horizontal shaking modes. The probable source is still rotor unbalance. However on Pump #2, notice the total pump assembly shows that the pump is rocking 180° out-of-phase in the vertical direction but has in-phase horizontal motion. This is the most typical situation for coupling misalignment.

Notice that in the next diagram, phase reading at Point C does not correspond to the shaking mode extension of readings at Bearings A and B. It looks as if the machine's frame must be twisting. Instead the "strange" phase angle at Point C is probably the result of a resonance at that end of the machine, in either the vertical or horizontal direction, of the suction pipe (or fan housing). Review how resonance (or partial resonance) affects phase in section "Vibration Phase Relative to Resonance."


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